This disclosure relates to optical windows having alternating positive refraction material layers and negative refraction material layers such that optical aberrations are minimized for optical transmission.
Optical windows are used in many applications, such as, for example, airborne electro-optical systems. For example, optical window may be used to isolate delicate optical sensors and/or other devices from the external environment (e.g., aerodynamic pressure, buffeting, particulate debris, salt spray, fungus and other contaminants, handling, and/or other factors), to provide a clear aperture with minimal optical aberration across the sensor field of travel and field of regard, to provide a smooth conformal surface to minimize aerodynamic drag impact on aircraft speed and range, to provide a conductive outer surface that is shaped to minimize the radar cross section of the air vehicle, and/or for other purposes. Conventionally, these are often conflicting requirements and may necessitate severe compromises between sensor performance, sensor durability, aerodynamic performance, radar observability, and/or other factors.
Typically, spherical transparent domes with concentric inner and outer surfaces are used on missiles and aircraft as optical windows to protect sensitive sensing equipment located inside. The axes of rotation for a gimbaled optical system within such domes typically intersect the center of curvature such that the optical power of the dome is the same for all pointing angles. These dome/window structures are typically made of a material with higher positive index of refraction than the surrounding medium (typically air) and, due to the curvature, distort optical rays propagating therethrough. To lowest order, the aberration may be in the focus term (i.e., referring to a Zernike polynomial decomposition of an optical aberration) with the window acting as a weak negative lens. However, higher order aberration terms may also be present, and become most severe as the aperture size approaches the inner dome diameter. The severity of the aberration is also dependent on the mismatch in refractive index between the dome material and the surrounding medium. For example, a transparent dome that has the same index as water will exhibit no refractive power in a water medium.
Geometrically conformal aerodynamic shapes may be desirable for aircraft window applications to minimize the aerodynamic drag, aero-optic boundary layer distortions, radar cross section (for stealthy airframes), and/or for other advantages. Ball-in-frustum and blunted ogive shapes may be desirable for missile domes for similar reasons. Unfortunately, conformal aerodynamic, ball-in-frustum, and blunted ogive shapes introduce severe optical distortions that are not the same for all pointing angles. Dynamic compensation optics and deformable mirrors have been used in the past to address this need, but these tend to increase the size and weight of the sensing system, reduce the reliability of the weapon system due to the number of additional moving parts (some of which need to remain precisely aligned) and control electronics, and increase cost due to the added complexity and number of additional precision optical elements.
Conventional approaches, such as that illustrated by optical window 100 in FIG. 1, teach the use of a single negative index compensating layer 102 that is physically thick (i.e., about a half of the entire windows thickness) abutted to a single positive index layer 104. Furthermore, the negative index layer is transparent for radiation at wavelengths to be compensated. Unfortunately, these requirements can be conventionally satisfied only for wavelengths in the microwave radiation domain. Applicants are unaware of any negative index materials or negative refraction materials known today for short optical wavelengths (e.g., ultraviolet, visible, infrared, and/or other short optical wavelengths) that can be physically thick (e.g., a few millimeters) and, at the same time, be physically strong and transparent. A failure to meet these three requirements makes the design of an optical window (e.g., an aircraft sensor window or missile dome) operating at the short optical wavelengths infeasible.
Another issue faced by conventional teachings is that, as the angle of incidence of an incident ray 106 (i.e., the angle between the incident ray 106 and the normal to the entrance surface of the negative index compensating layer 102) increases, refractive wedging occurs within one layer that is not compensated by the other. Also, the propagation distance through one layer is different than through the other and a boresight shift will result. As such, a given incident electromagnetic ray (e.g., the incident ray 106) will not be collinear and/or parallel with the corresponding transmitted electromagnetic ray (e.g., transmitted ray 108). It is noteworthy that the angular deviation and boresight shift in one conventional approach results, not from any theoretical problem in using negative index materials or negative refraction materials for refractive compensation, but from the thickness of the layers.